1. Field of the Invention
The present invention relates to epoxy resin compositions used for sealing semiconductor elements such as diodes, transistors, IC (Integrated Circuit), LSI (Large-Scale Integrated Circuit), etc.
2. Description of the Prior Art
Hitherto, in sealing a semiconductor element with a resin by transfer molding, there is a problem that strain arises inside the semiconductor elements when a rapid temperature change occurs during or after molding, due to the large difference between the thermal expansion coefficients of the inorganic material and the resin, and this easily induces large stress. Particularly, in epoxy resin compositions, because molding is generally carried out at a temperature as high as about 150.degree. to 190.degree. C., the resin, which has a larger thermal expansion coefficient compared to that of the semiconductor element, shrinks upon cooling to normal temperatures after molding and, consequently, the element covered with the resin receives a mechanical stress. This mechanical stress damages the semiconductor element such as by breaking or cracking and results in the occurrence of inferior goods.
On the other hand, as in the case of general molding materials, in the epoxy resin compositions used for sealing semiconductors, appropriate inorganic fillers are incorporated to reduce the cost and improve the molding operation by imparting a thixotropic property. Silica powder is a typical filler. This powder is most advantageous at present because it has excellent moldability and contains a lower amount of impurities which can adversely affect the semiconductor elements.
It has been generally known that when an inorganic filler is added to a resin composition, the thermal expansion coefficient of the molding resin decreases in proportion to the amount of the inorganic filler. From this viewpoint, it is believed that the addition of the silica powder to the epoxy resin composition reduces the difference in the thermal expansion coefficients of the molding resin and the semiconductor element, reduces the above-described mechanical stress and protects the semiconductor element from the damage of mechanical stress.
However, if the amount of the silica powder added is remarkably increased to attain such effects, it is possible to impair the moldability which is an inherent characteristic of the silica powder and also increase the modulus of elasticity of the molding resin. Since it is generally believed that the stress is proportionate to the product of the thermal expansion coefficient and the modulus of elasticity, increasing the amount of the silica powder does not necessarily result in reduction of mechanical stress, though the thermal expansion coefficient can be reduced.
As a result of investigating in detail the effects of the silica powder added in a generally admitted amount (about 50-85 wt%) in consideration of the above-described fact, it has been found that mechanical stress (measured by the method shown in the following examples) using a crystalline silica powder is about 1,800 kg/cm.sup.2 and, therefore, the occurrence of flawed goods such as breakage or cracking of the element is difficult to prevent. Further, in using an amorphous silica powder to reduce the thermal expansion coefficient, the mechanical stress is reduced to about 1,500 kg/cm.sup.2. In this case, the element is fairly improved compared to the case of using crystalline silica and the probability of the occurrence of inferior or flawed goods is reduced. However, the possibility of further improvement still remains.
In light of the above circumstances and as a result of studies to provide novel epoxy resin compositions capable of remarkably reducing mechanical stress without inhibiting the above-described inherent characteristics of the silica powder, the present invention has been accomplished.
The present invention is based on the discovery that the volume resistivity and the semiconductor characteristics are enhanced by using a specific inorganic filler in combination with the silica powder.